In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA) system, Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. User Equipments (UE) in those cells are served by respective radio base stations and are communicating with those respective radio base stations. The user equipments transmit data over an air interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data to the user equipments in downlink (DL) transmissions. When a user equipment moves from one cell to another the connection or link of the user equipment must be transferred in a so called handover (HO) process. Within for example LTE a reuse of frequencies between cells is used, similar to WCDMA-based systems. However, in LTE soft HO of a user equipment between cells is not a part of the standard specifications and instead hard HO of the user equipment between cells is adopted. Soft handover means that the user equipment is connected to both the cells simultaneously during the handover procedure. Hard handover means that a link to a source cell is released and then a link to the target cell is established. However, hard handover is a procedure which might experience failures such as Radio Link Failures (RLF).
Typical scenarios where RLFs might occur more often are the so called i) “high-speed train” and ii) “Manhattan” scenarios. A high speed train scenario is when a user equipment is moving fast between cells and a Manhattan scenario is when the user equipment is moving between a high number of cells due to a corner effect. A corner effect means that the user equipment may be served in one cell and when going around a corner a different cell may be in line of sight and the served cell may not have radio coverage around the corner leading to a very abrupt cell change. In order to combat RLFs, within present versions of LTE, such as Release 8 and beyond, a mechanism which permits the user equipment to recover from RLF has been defined.
The mechanism is termed “RLF recovery” within 3GPP and comprises a process wherein a radio link failure occurs during a handover at a time instance and the procedure to recover involves acquiring synchronization with the cellular system again. The synchronization requires a certain time to be performed. After that, the user equipment selects a best cell in terms of measuring signal strength such as Reference Signal Received Power (RSRP) or Reference Signal Received Power Quality (RSRQ) which are processes to indicate signal strength of reference signals. This also requires some time interval to be performed. The user equipment then performs a random access procedure also requiring some time. The random access procedure is performed in order to establish a connection. Finally, the user equipment performs a Radio Resource Control (RRC) Connection Reestablishment Request and receives a RRC Connection Reestablishment Complete response indicating connection reestablished.
A slow RLF recovery implies a long interruption time. The duration of the whole radio link failure recovery is dictated by a timer. In case the user equipment does not receive an acknowledgement (ACK) such as an RRC Connection Reconfiguration Request ACK message from the network within a time duration defined by the timer, then the RLF recovery procedure is determined to have failed and the user equipment goes from an active mode back to an idle mode. In this case, the RRC connection is lost and the user equipment needs to establish a new RRC connection. Simulations have shown that in these challenging mobility scenarios, the whole procedure lasts 500-600 ms for 90% of the cases.
The time duration for performing the RRC connection reestablishment may increase the interruption time. Prior art solutions describe scenarios where the cell that receives the RRC Connection Reestablishment Request message contacts the last serving cell of the user equipment in discussion here. This is a feasible option since the RRC Connection Reestablishment Request message includes the temporary cell id of its last serving cell. This range of solutions is termed within 3GPP as “UE context-fetching” solutions. Prior art solutions are solutions for maintaining the RRC connection, since the RLF recovery procedure is successful within the time specified by the timer ruling this procedure. However, prior art solutions imply that the interruption time remains at a high level; as the cell where the UE has landed during RLF recovery has to communicate with the last serving cell of the UE and fetch its context. This procedure of fetching the user equipment context involves communication via X2, which requires some time. For non-delay critical services, this might generate problems in the Transmission Control Protocol (TCP) execution and for delay-critical services, this interruption time is perceived by the user equipment and may be perceived as annoying by a user of the user equipment.